With some fabrics, especially of natural origin repeated washing can lead to fabric harshness, giving the fabrics an unpleasant feel. For some years, fabric conditioning products have been available, intended inter alia for alleviating this fabric harshness by softening the fabrics in a post-washing step, for example in the rinse step of a fabric laundering process. There has been a desire to provide a single detergent composition which would be capable of both washing and softening the fabrics to overcome the inconvenience of using separate products.
According to British Patent GB 1 400 898 (Procter and Gamble/STORM) a possible solution to this problem is to include in the detergent composition a smectite clay containing material having a cation exchange capacity of at least 50 meq/100 g, together with a detergent active material.
While some success has been obtained with the use of such clay materials, softening performance still does not generally match that obtained by the use of separate products and there is therefore scope for improving performance.
Clays of interest in the present invention are swelling types, which expand and delaminate in liquid media. These clays belong to the group of phyllosilicates and are three-layer sheet type crystalline materials. The sheet structures are composed of three layer arrangements of tetrahedral silica, octahedral alumina, tetrahedral silica. The central layer may be dioctahedral or trioctahedral and the three layer sheet structures are separated by an interlamellar space.
The layer structure is subject to many variants in nature. For example the central octahedral layer may be occupied not by two aluminium ions (Al.sup.3+) (dioctahedral) but by three magnesium ions (Mg.sup.2+) (trioctahedral). In dioctahedral clays the octahedral layer may be partially occupied by the substitution of one Al.sup.3+ for one Mg.sup.2+ resulting in a residual surplus of negative charge in the structure. A residual surplus of negative charge can also arise when silicon ions (Si.sup.4+) in the tetrahedral layer are replaced by aluminium ions (Al.sup.3+).
The surplus of negative charge requires the presence of balancing cations which are located in the interlamellar space between the sheet structures. A measure of the degree of the surplus charge is given by the number of exchangeable cations, as reflected by the cation exchange capacity CEC of the pure mineral. The CEC of a mineral is directly related to the lattice charge deficiency of that mineral.
This can be further explained by a general representation of clays useful in the process of the present invention which fall into the half unit cell formula: ##STR1## where X.sup.n+ is a balancing exchangeable cation for example Na.sup.+, Mg.sup.2+, (Ca.sup.2+ or K.sup.+ which can be univalent (n=1) or divalent (n=2)).
y+b is the lattice charge deficiency of the mineral per half unit cell PA1 M.sup.III is a trivalent metal ion e.g. Al.sup.3+, Fe.sup.3+, Cr.sup.3+ PA1 N.sup.II is a divalent metal ion e.g. Mg.sup.2+, Fe.sup.2+, Ni.sup.2+, Zn.sup.2+ PA1 y is a positive number less than four, or zero PA1 a is a positive number less than or equal to two and PA1 b is a positive number less than two, or zero.
A unit cell is the smallest group of ions or atoms whose repetition at regular intervals, in three dimensions, produces the lattice of a given crystal.
CEC measurements indirectly determine the number of ##STR2## present in 100 g and quote these as meq. The value of y+b (the lattice charge deficiency) in gramme equivalents per half unit cell is therefore directly related to CEC.
The smectite clays taught in GB 1 400 898 are 2:1 layer phyllosilicates which we believe are characterised by possessing a lattice charge deficiency in the range of 0.2 to 0.6 g equivalents per half unit cell.
European Patent Application 0 297 673 (Procter and Gamble/BAECK) discloses that the softening performance of detergent compositions can be improved by increasing deposition of clay onto the fabric during the washing process. Increased deposition is said to be achieved by the inclusion of from 1 to 25% by weight of a natural hectorite clay in the form of particles in the composition where at least 50% of the clay we believe has a value of y+b from 0.23 to 0.31.
The number and availability of such naturally occurring clays is fairly limited. Such naturally occurring clays may also be of unsuitable colour for inclusion in a detergent composition or may give rise to fabric colour dulling once deposited thereon.